FIELD OF THE INVENTION
The present invention relates to a process for preventing scale deposition in a water washing type collector and pipe line instruments thereof in an operation for feeding a gas obtained by a catalytic gas phase oxidation of naphthalene into the water washing type collector to collect naphthoquinone and phthalic anhydride in the gas as a slurry of naphthoquinone and phthalic acid with an aqueous medium.
It has been known to use an atomizer, a scrubber or a bubble tower as a water washing type collector for collecting napthoquinone and phthalic anhydride from a gas obtained by a catalytic gas phase oxidation of naphthalene by washing with an aqueous medium by an industrial operation. In the conventional process, scale is deposited in a water washing type collector and pipe line instruments thereof, so as to form a deposited layer having a thickness of several tens to several hundreds mm and to decrease an effective capacity of the collector and the pipe line instruments. The scale is peeled off and accumulated in the tower and the vessel and is flowed into the pipe and a pump whereby the flow rate becomes irregular and the devices are clogged. Thus, serious troubles are caused such as the continuous operation can not be carried out and the devices are damaged. It has been well-known that the scale deposition in the water washing collector and the pipe line instruments thereof causes serious industrial disadvantages (such as Japanese Patent Publication No. 9209/1978). Moreover, a large amount of naphthoquinone remains as the scale in the devices so as to cause inferior yield of naphthoquinone. Thus, the scale should be peeled out and naphthoquinone should be recovered. However, the solubility of naphthoquinone in water is relatively low and naphthoquinone is easily decomposed by heat or an alkaline solution, and accordingly, the dissolution and the separation by using hot water or an alkaline aqueous solution is industrially difficult and disadvantageous. Moreover, naphthoquinone causes irritation of skin and accordingly, a manual operation for discharging scale containing a large amount of naphthoquinone is quite difficult. Thus, when the separation of the scale is indispensable, the scrubber and the recycling system should be washed with an alkaline aqueous solution. In the separation, the operation would be interrupted disadvantageously.
It has been proposed to improve the separation of the scale deposition as follows.
(1) The collection is carried out as an aqueous solution of a monophthalate having pH of 3 to 5 by adding a base in a solution for water washing (Japanese Patent Publication No. 9209/1978).
(2) The collection is carried out by using a dilute aqueous solution of phthalic acid or an aqueous solution of maleic acid.
The process (1) is not economical because a base is used and an acid for neutralization is required for recovering phthalic acid. The process (2) is not practically employed because a large amount of the aqueous solution is formed.
The inventors have studied the reason why the scale deposition is caused in detail in order to prevent the scale deposition in the water washing collector and the pipe line instruments thereof and to maintain the stable operation of the collector.
According to the analysis of the scale deposited, it has been found that more than 90% of the total component of the scale is naphthoquinone even though naphthoquinone and phthalic acid are in the form of an aqueous slurry.
According to an analysis of components dissolved in a filtrate obtained by a filtration of the aqueous slurry, it has been found that the filtrate is in a supersaturated condition, that phthalic acid and naphthoquinone are dissolved over their solubilities at the temperature, and the scale deposition caused is substantially proportional to the supersaturated degree.
The inventors have studied to reduce the supersaturated degree in view of the above-mentioned findings. That is, various tests have been made by adding various additives to the aqueous slurry containing naphthoquinone and phthalic acid obtained by the water washing collection. It has been found that there is substantially no effect by an addition of fine powdery naphthoquinone but when a fine powdery phthalic acid is added, the supersaturated degree of phthalic acid and naphthoquinone dissolved in the liquid phase could be decreased even though solid phthalic acid is in the slurry.
Thus, a fine powdery phthalic acid in a slurry form is added to the slurry in the water washing type collector, whereby the supersaturated degree is decreased and the scale deposition in the apparatus could be substantially prevented. The present invention has been completed by the findings.